Preheat oven to 375°F. Place the sugar and water in a 9.5-inch non-stick skillet over low heat and cook, stirring, until the sugar is dissolved. Increase the heat to medium and bring to a boil. Cook, without stirring, for 7 to 9 minutes or until light golden. Carefully add the butter and stir until melted and well combined.

Remove the pan from the heat and carefully arrange the apples, cut-side up and slightly overlapping, in the caramel. Top the apples with the pastry round and fold the edges under to tuck in the apples. Using a fork, make a few marks in the center of the pastry. Bake for 35 to 40 minutes or until the pastry is puffed and golden. Allow to stand for 2 to 3 minutes. Carefully turn over the tart on a plate to serve.